unp_externalize is responsible for externalizing the file descriptors carried within a unix domain socket message.
That means allocating new fd table entries in the receiver and recreating a file which looks looks (to userspace) like the file
the sender sent.

Here's the relevant code:

  for (i = 0; i < newfds; i++) {     <----------- (a)
#if CONFIG_MACF_SOCKET
    /*
     * If receive access is denied, don't pass along
     * and error message, just discard the descriptor.
     */
    if (mac_file_check_receive(kauth_cred_get(), rp[i])) {
      proc_fdunlock(p);
      unp_discard(rp[i], p);
      fds[i] = 0;
      proc_fdlock(p);
      continue;
    }
#endif
    if (fdalloc(p, 0, &f))
      panic("unp_externalize:fdalloc");
    fp = fileproc_alloc_init(NULL);
    if (fp == NULL)
      panic("unp_externalize: MALLOC_ZONE");
    fp->f_iocount = 0;
    fp->f_fglob = rp[i];
    if (fg_removeuipc_mark(rp[i]))
      fgl[i] = rp[i];
    else
      fgl[i] = NULL;
    procfdtbl_releasefd(p, f, fp);
    fds[i] = f;
  }
  proc_fdunlock(p);                <-------- (b)

  for (i = 0; i < newfds; i++) {
    if (fgl[i] != NULL) {
      VERIFY(fgl[i]->fg_lflags & FG_RMMSGQ);
      fg_removeuipc(fgl[i]);      <--------- (c)
    }
    if (fds[i])
      (void) OSAddAtomic(-1, &unp_rights);
  }


The loop at a gets the fileglobs from the socket message buffer and allocates new fds in the receiver
and sets the fp->f_fglob fileglob pointer to point to the sent fg. After each new fd is allocated and initialized
the fd is released and associated with the new fp.

At (b) the code then drops the process fd lock at which point other threads can access the fd table again.

At (c) the code then removes each of the fileglobs from the list of in-transit fileglobs; however this list *doesn't*
hold an fg reference therefore there's nothing stopping the fg from getting free'd via another thread closing
the fd between (b) and (c).

Use zone poisoning for a reliable crasher.

tested on MacOS 10.12.3 (16D32) on MacbookAir5,2 
 

domain_sock.c 6.6 KB View Download